A brief acoustic stimulus (prepulse) which is not itself able to elicit a startle response in rats can inhibit or facilitate startle to a loud sound (pulse) which it leads. With very short prepulse-pulse intervals (5 ms), startle latency is shortened and with longer intervals (approximately 40-4000 ms), startle latency is lengthened and amplitude is reduced. We have found that the eyeblink response in human subjects also shows the inhibitory effect at 200 ms but, at 800 ms and at longer intervals, facilitation occurs. The proposed work will study the blink response to determine whether the facilitation-inhibition-facilitation effects can be dissociated, i.e. whether they are differentially affected by various characteristics of stimulation and are differentially present during sleep and early development. It is hypothesized (1) that onset characteristics of the prepulse determine whether a startle center is primed and early facilitation occurs, (2) that steady-state characteristics activate an arousal mechanism leading to inhibition, and (3) that informational characteristics involve cortical processing which determines the long-ISI effects.